Since lithium secondary batteries in which the dope-undope phenomenon of lithium is utilized exhibit high energy densities, as cellular phones, personal computers and the like have been downsized, they have been widely spread in the fields of communication appliances and information-related appliances. Moreover, in the field of automobiles as well, it has been urged to develop electric automobiles because of the environmental problems as well as the resource problems, as an electric source for electric automobiles as well, lithium secondary batteries have been investigated.
Lithium secondary batteries, which have been currently put to practical use, are generally constituted by a positive electrode which uses a lithium-transition metal composite oxide as a positive electrode active material, a negative electrode which uses a carbon material and the like as a negative electrode active material, and a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent, and those which exhibit high voltages of 4 V-class make a main stream.
However, since the above-described lithium secondary batteries use nonaqueous organic solvents whose burning points are low as the electrolytic solutions, the safety matters. For example, in case of arriving at over charged states, and in case of being exposed to high temperature environments, for the purpose of securing safety, it is general to equip them with devices such as PTC elements and safety valves. However, since combustible solvents are used, in order to fully secure safety, considerable difficulties follow them about. In particular, secondary batteries as an electric source for powering automobiles and the like are big, and the amounts of used organic solvents are large, in addition thereto, it is expected to use them under severe conditions such as the service temperatures, much higher safety is required.
Moreover, when moisture is present even in a small amount in batteries, there arise various problems such as the generation of gases by means of the electrolysis reaction of water, the consumption of lithium by means of the reaction between water and lithium, the corrosion of battery constituent materials. Accordingly, in the production of lithium secondary batteries, a thoroughly dry environment is required, special equipment for completely removing moisture and a large amount of labor are needed, and this is one of the causes for pushing up the costs of batteries.
Meanwhile, in aqueous lithium secondary batteries which use aqueous solutions as the electrolytic solution, the aforementioned problems do not arise basically. Moreover, in general, since aqueous solutions are of better conductivity compared to nonaqueous solutions, the reaction resistance of batteries also decreases, and the power characteristic and rate characteristic of batteries are improved. However, since it is necessary to charge and discharge in a potential range where the electrolysis reaction of water does not occur, the aqueous lithium secondary batteries suffer from a drawback in that it is difficult, compared to nonaqueous lithium secondary batteries, to secure a large discharge capacity.
From this, in aqueous lithium secondary batteries, it is desired to use an active material which is not only stable in aqueous solutions but also can reversibly dope-undope lithium ions in a large amount in a potential range where oxygen and hydrogen are not generated by means of the electrolysis of water, namely which exhibits a large capacity.
As for aqueous lithium secondary batteries which have been conventionally investigated, there exist, for example, as disclosed in Published Japanese Translation of PCT International Publication for Patent Application No. 9-508,490, a battery which uses LiMn2O4 and the like as the positive electrode active material and LiMn2O4, VO2 and so forth as the negative electrode active material, and, moreover, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 12-77,073, a battery which uses LiCoO2, Li(Ni, Co)O2, LiMn2O4 and the like as the positive electrode active material and LiV3O8 and so forth as the negative electrode active material.
When the present inventors carried out various tests while paying attention to the active materials, it was found out that it is difficult for LiCoO2, Li(Ni, CO)O2, LiMn2O4 and the like, which are positive electrode active materials having been investigated conventionally, and for LiV3O8, VO2 and so forth, which are negative electrode materials, to take out a sufficient capacity in a potential range where the electrolysis reaction of water does not occur, and that they further have a problem as well in terms of the stability in aqueous solutions. Therefore, in case of actually constituting aqueous lithium secondary batteries by using them, the capacities and cycle characteristics of the lithium secondary batteries do not become practically satisfactory ones.